Identifying Opportunities for Improving Water Use Efficiency, 2016

 

Project Leader

Bruce Linquist, UCCE rice specialist, Dept. of Plant Sciences, UC Davis

The goal of this project is to identify opportunities to conserve water in California rice systems. Objectives of 2016 research were:

  • Determine how water temperature affects crop development.
  • Complete analysis of salinity research and develop salinity management guidelines.
  • Initiate studies on groundwater levels and implications for irrigation management.
  • Air/water temperature trials

    Air temperature is commonly used to predict plant development in most crops, including rice. However, in flooded rice systems, the growing point of the plant is under water until mid-booting stage—about 70 days into the season. Thus, it is reasonable to expect that water temperature is a more important driver of crop development.

    Two sets of field trials with M-206 were used to evaluate the effects of air and water temperature on rice crop development. The first was a cold water gradient study conducted in 2014 to directly analyze the effect of air and water temperature on crop development. Transects were set up to monitor soil, water, and air temperature with sensors located at the inlet to the first check. At each sensor, the date of panicle initiation, heading, and maturity were recorded.

    The second set of field trials was part of the statewide variety trials. Data from this trial were used to validate a model developed from the cold water gradient study. Results showed that panicle initiation, heading, and physiological maturity can be delayed. This confirms that water management—because of differences in air and water temperature—plays an important role in rice development and degree-day accumulation. Rice crop development models based solely on air temperature will always be somewhat limited in their accuracy.

    Salinity in no-spill systems

    Drought conditions in recent years resulted in some water districts requiring no-spill water management. Given interest in evaluating opportunities to reduce water use, research was begun in 2014 to evaluate the impact of no-spill water management on salinity buildup and yield. This research expanded in 2015 with different fields under both no-spill management and continuous flow-through systems.

    Researchers evaluated soil salinity and how it varies across the season and affects rice yields. Results show a consistent pattern of floodwater salinity buildup throughout the season. Floodwater salinity levels are highest early in the season and farthest from the inlet.

    In most California rice fields, salinity should not be a problem, as electroconductivy (a measure of salinity) in soil and water is low. In fields that receive high salinity water or have soils that are high in electroconductivity, maintenance flows may help somewhat. A better strategy is to have open weirs on either side of the field and switch occasionally during the season. This helps flush out stagnant water that may be higher in salinity.

    The pattern of soil solution salinity varied more than the pattern of floodwater salinity, and largely depended on the initial level of salinity in the field. Soil solution salinity was consistently greater in bottom checks.

    Groundwater/floodwater study

    During the 2016 season, researchers monitored subsurface hydraulic head gradients in three commercial rice fields to better understand groundwater/floodwater interactions. Monitoring wells called piezometers were installed at three different depths in the soil profile to monitor hydraulic head at each depth. The direction of water flow can be determined based on differences in hydraulic head.

    Percolation and seepage rates at all three fields were found to be very low. Percolation rates averaged about 1.1 inch and seepage rates about 1.8 inch for the season. However, the rates were highly variable.

    Results from the piezometer studies also indicated that surface floodwater moved downward during the season, indicating percolation. There was also indication that in some fields water was moving upward from the groundwater table.

    The objective of this study was to determine if the height of the groundwater table influenced the rates of percolation. There were too few sites in 2016 to get a good sense of this. However, the methodology has been established and more research in this area is planned for 2017.

    Monitoring salinity

    Eleven field sites throughout the Sacramento Valley were monitored weekly for salinity during the 2014 and 2015 growing seasons. In each field, three study plots were established in each of the top, middle, and bottom checks—one plot close to the water inlet, one plot in the middle, and one at the farthest point from the water inlets.

    Water salinity measurements were collected at all plots. Soil solution salinity measurements were collected at circled plots. The solid line and arrow on the left represents the distance down the field, while the dashed line and arrow in the middle represents the distance across the field.